Conventionally, when a multilayer substrate (HDI) is manufactured in which high-density connection is realized by formation of via holes, in formation of an insulating layer of the multilayer substrate, a prepreg and a copper foil are laminated and cured by vacuum press or the like, the copper foil in a via hole forming portion is removed by etching or the like, and, then, the via holes are formed by laser machining. Alternatively, surface treatment is applied to the copper foil and the via holes are directly formed by the laser machining.
As another method of forming via holes, there is a method of uniformly applying varnish-like ink resin over the entire both surfaces of a rigid core substrate with screen printing, a curtain coat, a roll coater, a spray, or the like and, then, forming the via holes with exposure and development (when the resin is photosensitive) or forming the via holes with a laser in the same manner as the prepreg (see, for example, Patent Document 1).
However, in these methods, when the laser is used, an expensive laser machining apparatus is necessary. Some laser machining conditions may cause a connection failure. When photosensitive resin is used, the resin does not always have a sufficient characteristic as a material to be formed as an insulating layer of a substrate and does not have a degree of freedom of material selection. Photosensitive resin that can be used for the substrate is generally expensive.